Analog-digital converters (A/D converters) are used in integrated circuits for the conversion of analog input signals into digital values. The clocked converters sample the input signal at different times and generate digital values from these samples. For input signals at high levels in the range of several tens of volts, these converters should be manufactured using special production techniques, for example, in a multi-film oxide CMOS process. Analog-digital converters produced in this way are also suitable for signals with exceptionally high input levels in contrast to converters using conventional production technology. Conventional converters could overload at high input signal levels and thus deliver false values or could even be damaged. However, one disadvantage of this multi-film oxide CMOS converter is that the high withstand voltage is achieved only at the expense of space requirements and the conversion rate.
One alternative to the voltage-resistant analog-digital converters consists in attenuating the input signal by a defined factor and feeding the attenuated signal to a conventional analog-digital converter arrangement. The A/D converter contained in this conventional converter arrangement then can be produced using CMOS technology and has low space requirements with a simultaneously high conversion rate. The attenuation of the input signal by a constant factor, however, can lead to problems for input signals with low levels. These are attenuated again so that error during the analog-digital conversion is amplified due to inherent noise within the converter. Under some circumstances, the attenuation can have the result that the level of the supplied, attenuated input signal is smaller than a quantization step of the A/D converter. In other words, the constant attenuation limits the resolving power of the converter for low input signals.
The publication U.S. Pat. No. 5,861,831 and FIG. 9 show a converter arrangement with an active circuit for attenuating an input signal. The arrangement of an active circuit shown in FIG. 9 is realized by an adjustable amplifier V. This is connected to the input of the converter ADC by means of a filter. The amplification factor is set here by a level detector PD to which the signal output by the amplifier is fed. Another level control signal allows a desired setting of the detector PD. Furthermore, a time circuit TC is provided that controls both the actual A/D converter ADC and also the peak detector PD. The signal output by the amplifier V is fed via a filter to the A/D converter ADC. The detection of signal changes in the input signal is limited by the bandwidth of the peak detector PD. The illustrated converter is suitable primarily for converting intermediate frequency signals. In addition, due to the additional level detector PD, the current consumption of the entire arrangement is increased.